In motor vehicles in particular, pumps are used for a variety of purposes. For example, an oil pump is used to ensure an oil supply, for example for the lubrication of the internal combustion engine or of the transmission.
Here, use is often made of a pump that exhibits a constant volume flow, which pump generates a volume flow that can satisfy the maximum and minimum conditions.
If said pumps are driven by the internal combustion engine for example via a belt drive, the rotational speed of the drive of the pump also varies, such that the minimum demands on the volume flow must be satisfied at the lowest rotational speed, whereas at high rotational speeds, the maximum volume flow must be attained in order to realize the volume flows required in this operating situation.
In the case of constant drive rotational speed, however, the volume flow cannot be adjusted.
If use is made of a fully variable vane-type pump, the stop for the minimum delivery volume flow is selected such that a minimum delivery is ensured, because a minimum delivery volume flow is always required since this is required for pressure generation.
If said pump is supplemented by a gearwheel pump connected in parallel, said gearwheel pump assists in the delivery of the volume. During cold operation, however, more fluid is delivered than is required by the internal combustion engine of the vehicle in order to attain the demanded pressure. This would, in the case of cold temperatures, result in the fluid pressure, for example oil pressure, being higher than required, which would have a disadvantageous effect on the drive power and exhaust-gas emissions. Furthermore, a cut-off valve would have to be designed for the conditions during continuous operation, because it has the task of limiting the pressure not only during starting operation when the engine is cold but also at high engine speeds under cold conditions. This is however disadvantageous and expensive for the configuration of the valve.
It is therefore the object of the present invention to provide a pump by means of which a variable oil supply can be ensured, wherein the pump should nevertheless be of simple and inexpensive construction.